1. Field of the Invention
The present invention relates to an antenna device provided for a radio communication device, such as a mobile terminal, and more specifically to an unbalanced feeding antenna device for making radio communications among a plurality of radio systems each having different frequency bands.
2. Description of the Related Art
In recent years, in a mobile terminal typified by a mobile phone and a personal digital assistant (PDA), multi-functionality has been developed, wherein the mobile terminal mounts an interface for a short-distance radio system, such as a wideband local area network (LAN) and Bluetooth (registered trademark), a terrestrial digital broadcast receiver, etc., in addition to a standard mobile communication interface. Furthermore, mounting of a new radio interface for an ultra-wide band (UWB), etc., has been examined for the future.
In general, such a type of mobile terminal mounts antennas exclusive for each of a plurality of radio interfaces or intends to correspond to each radio interface by achieving multi-frequency of a single antenna. However, in such configuration, deterioration in performance caused by an increase in antenna mounting volume and inter-antenna interference is a possible risk.
Therefore, conventionally, a single antenna shared for a plurality of radio system is disposed and a plurality of matching circuits corresponding to each of the plurality of the radio systems are disposed between the shared antenna and a radio circuit module. An antenna device for selecting a matching circuit corresponding to a radio system to make communication and obtaining optimum impedance matching is proposed by disposing each changeover switch at opposed ends of a matching circuit group, respectively, and by changing over these switches (for instance, refer to Jpn. Pat. Appln. KOKAI Publication No. 2003-347959).
However, the above-mentioned antenna device disposes each changeover switch at the opposed ends of the matching circuit group, respectively. Thereby, the antenna device causes a reduction in antenna radiation efficiency because of an increase in loss due to the changeover switches as well as increasing cost. Particularly, in the event of an arrangement of the matching circuit group near by the antenna, a current magnitude becomes a maximum amount in the vicinity of a connecting point between the antenna and the matching circuit group. Therefore, it is not highly desirable for the antenna device to arrange the changeover switches at that connecting point because the loss due to the changeover switches becomes extremely large.